1. Field of the Invention
The present invention relates to an apparatus for detecting a focusing error, and more particularly, to a focusing error detecting apparatus for detecting a focusing error of an objective lens, with respect to the recording surface of an optical disk, which is incorporated in an optical disk apparatus for optical information reproduction.
2. Description of the Related Art
In optical disk apparatuses for reproducing from and recording information on, e.g., an optical disk, an objective lens, used to converge a light beam from a light source on the recording surface of the disk, must be kept focused so that the smallest beam spot is formed on the recording surface, during recording and/or retrieving operation. In consideration of this requirement, these optical disk apparatuses incorporate a focusing servo unit, which detects a focusing error of the objective lens with respect to the recording surface, and moves the objective lens in the direction of its optical axis in accordance the detected focusing error.
The double-knife-edge method, among many others, is conventionally known as a typical method of focusing error detection in the optical disk apparatuses.
FIG. 1 shows an arrangement of an optical system of a focusing error detecting apparatus which uses the double-knife-edge method. A light beam reflected from an optical disk (not shown in FIG. 1) passes through an objective lens (not shown) in the opposite direction to a light beam directed to the disk, and is separated from the beam bound for the disk by beam splitter 101. Then, the light beam is divided into two light beams by wedge prisms 102 and 103 (or split prism), and the split beams are converged, by converging lens 104, on detecting regions 105a, 105b, 106a and 106b of two photo-detectors 105 and 106, which are located in different positions.
FIGS. 2A to 2C show how the spot shapes of the light beams on detecting regions 105a, 105b, 106a and 106b of photodetectors 105 and 106 change, and FIG. 3 shows characteristic curves representing changes of a focusing error signal and a retrieving information signal with respect to a focusing error. If output signals corresponding to detecting regions 105a, 105b, 106a and 106b of photodetectors 105 and 106 are A, B, C and D, respectively, the focusing error signal, indicative of the focusing error of the objective lens, and the retrieving information signal, both produced by a processing circuit, are given by {[A+D]-[B+C]}and [A+B+C+D], respectively. According to the double-knife-edge method, as seen from these drawings, the level of the focusing error changes substantially near the focused point, and the sensitivity for focusing error detection is high.
In the conventional focusing error detecting apparatus, however, if the objective lens is kept focused so that the smallest spots are formed on the optical disk, very small circular spots are formed on the photodetectors, as shown in FIG. 2B, and most of beam is falls into non-detecting regions of photodetectors 105 and 106. In the focused state, therefore, the intensity of the light beams detected by means of photodetectors 105 and 106 is very low. Also, the width of the non-detecting region of each photodetector is relatively wide, and if most of the beam spots are formed within the non-detecting regions of photodetectors 105 and 106 the level of signals detected by the photodetectors vary considerably. As a result, the focusing error signal {[A+D]-[B+C]} is extremely unstable. The level of the retrieving information signal [A+B+C+D] is so low in the focused state, moreover, that the signal-to-noise ratio of the retrieving information signal is not satisfactory.
The shape of the spot shown in FIGS. 2A to 2C is very sensitively changed depending on the focusing error so that the range of focusing error detection is very narrow. Therefore, it is difficult property to cope with a substantial focusing error which may be caused when the objective lens is withdrawn in the initial stage of focusing servo operation, or in normal recording and/or reproducing operation.
In the focusing error detecting apparatus based on the conventional double-knife-edge method, as described above, the intensity of the light beam detected by each photodetector in the focused state is reduced depending on the width of the non-detecting region of the photodetector. Thus, steady focusing error detection cannot be easily effected, the retrieving information signal cannot enjoy a satisfactory signal-to-noise ratio, and the range of focusing error detection is narrow.